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SubjectRe: dm: add secdel target
On Thu, Oct 18, 2018 at 04:01:25PM -0400, Mike Snitzer wrote:
> On Sun, Oct 14 2018 at 7:24am -0400,
> Vitaly Chikunov <vt@altlinux.org> wrote:
>
> > Report to the upper level ability to discard, and translate arriving
> > discards to the writes of random or zero data to the underlying level.
> >
> > Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
> > ---
> > This target is the same as the linear target except that is reports ability to
> > discard to the upper level and translates arriving discards into sector
> > overwrites with random (or zero) data.
>
> There is a fair amount of code duplication between dm-linear.c and this
> new target.
>
> Something needs to give, ideally you'd factor out methods that are
> shared by both targets, but those methods must _not_ introduce overhead
> to dm-linear.

I see three possible solutions:
- integrate 'secdel' into dm-linear;
- export functions like linear_dax_copy_to_iter and fill them into
secdel's `struct target_type`;
- call methods of linear's `struct target_type` from secdel's one.

> Could be that dm-linear methods just get called by the wrapper
> dm-sec-erase target (more on the "dm-sec-erase" name below).

> > diff --git a/drivers/md/dm-secdel.c b/drivers/md/dm-secdel.c
> > new file mode 100644
> > index 000000000000..9aeaf3f243c0
> > --- /dev/null
> > +++ b/drivers/md/dm-secdel.c
> ...
> > + if (mode == SECDEL_MODE_RAND) {
> > + page = alloc_page(gfp_mask);
> > + if (!page) {
> > + DMERR("%s %lu[%lu]: no memory to allocate page for random data",
> > + __func__, sector, nr_sects);
> > + /* will fallback to zero filling */
>
> In general, performing memory allocations to service IO is something all
> DM core and DM targets must work to avoid. This smells bad.
>
> ...

Yes, but we need to create and write random data, so I don't see other
solution. In zero overwriting mode page is not allocated. And it's fall
back to zero overwriting if page allocation fails for random data.

> Mike

I will address these and other your concerns in v2.

Thanks!

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